Whisk 1/2 cup milk and egg yolks in medium bowl. Bring 1 1/2 cups milk, cream, sugar, and salt to simmer in heavy large saucepan over medium-high heat, whisking until sugar dissolves. Gradually whisk hot milk mixture into yolk mixture; return to same saucepan. Stir over medium heat until custard thickens enough to leave path on back of spoon when finger is drawn across, about 6 minutes (do not boil). Strain custard into medium bowl.
Place over large bowl of ice water and cool, stirring often.
Whisk in crème fraîche and vanilla. Process in ice cream maker according to manufacturer's instructions.
Transfer to container. Cover and freeze at least 6 hours and up to 3 days.
Stir sugar and 1/2 cup water in heavy medium saucepan over low heat until sugar dissolves, occasionally brushing down sides of pan with wet pastry brush. Scrape in seeds from vanilla bean; add bean. Increase heat and boil without stirring until syrup is deep amber color, occasionally brushing down sides and swirling pan, about 16 minutes (time will vary depending on size of pan).
Remove pan from heat.
Mix in cream, crème fraîche, and butter (mixture will bubble vigorously). Stir until sauce is smooth. Discard vanilla bean. (Can be made 3 days ahead. Cover and chill. Rewarm slightly before serving.)
Preheat oven to 350°F.
Mix cashews, oil, and salt in small bowl.
Spread out on rimmed baking sheet.
Bake until nuts are golden, stirring often, about 12 minutes. Cool. (Can be made 1 day ahead. Store airtight.)
Scoop ice cream into dishes. Spoon warm caramel sauce over. Top sundaes with whipped cream and cashews.
